As the proportion of blast furnace charge compositions in pelletized form of iron ores increases due to the excellent adaption of pellet properties thereto, increasing requirements are directed not only to the feature that the processed pellets, being regarded merely as a mass of particles, possess an excellent overall quality with respect to crushing strength, reducibility and reduction swelling, but also the feature that the individual pellets have qualities with as small an unevenness as possible. It is known that the quality of a pellet depends upon many factors, for example, the pelletizer charge composition (blend ratio of different source iron ores, mineralogical composition, chemical composition and size distribution), pretreating parameters (moisture content and mixing time), pelletizing parameters (rate of rotation of the pelletizer, inclination angle, volumetric depth and average retention time), drying parameters (temperature and rate), firing parameters (heat pattern and atmosphere) and cooling parameters (temperature and rate); these factors influencing one another in a complex manner.
Prior art pelletizing systems have generally ignored variations in charge composition which may occur in a short period of time, for example, in a single production run, and have permitted for the process operator to determine the control factors in the subsequent steps in the process, assuming that the initial charge composition does not vary. Even if samples or assay were periodically cut from the process stream on a short time interval cycle in sequence, the assay information would not be available until from several hours in the swift case and usually not until the next day. By this time, the parameters of the process had, in all likelihood, altered so significantly that any changes made in the processing based on the sample assay results would be useless.
On the other hand, more recently, there has been an increasing tendency to use finely-divided iron ores which are transported in slurry form and stored in the settling pond, and dry powder of iron ore after the screening thereof has been seen in making blast furnace charge composition. Further, it has become a frequent practice to blend a plurality of iron ores of different sources and additional ingredients such as lime and dolomite in various ratios. As a result, it is practically impossible to avoid some variations with time of size distribution and compositional ranges of the constituents in the composition which is to be supplied to a pelletizer.
FIG. 1 shows an example of such variations in percentage of particular size particles and a constituent, in this instance, SiO.sub.2, in the starting material used in a pellet manufacturing plant. This problem can not be solved unless further information than the results of the chemical analysis and size distribution assay made at such a low frequency as one time a day or a production run are provided. Even if the sampling frequency is increased, the prior art system cannot control the pelletizing and pellet processing operation in any optical fashion because of the high rate of processing and the multiplicity of control factors as has been described above.
An investigation of the present inventors has indicated that fired pellets as the final product, though sampled from the same production run, have qualities different from one another, which qualities are distributed over a considerably wide range. These qualities are related to the apparent specific gravities (or apparent densities) of the respective pellets as shown in FIG. 2. Further investigations have led us to a finding that the reason why the qualities of the fired pellets are distributed over such a wide range is based on the fact that insufficient pretreatment of the pelletizer change composition and inappropriate control of the pelletizing parameters impart different apparent specific gravities to different green pellets which are to be presented to a firing furnace, and that such a distribution range is further extended by heat pattern variation which may occur in the firing furnace.